This invention relates to thermally tunable optical waveguide devices of enhanced stability and to optical communication systems employing them.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals, and fiber amplifiers for amplifying the signals along the transmission path. It also includes a receiver for detecting the signals and for demodulating the information they carry. Increasingly, the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Practical systems must also include a variety of tunable or switchable components for compensating the chromatic dispersion of transmitted signals, routing signals among different paths in the network and compensating nonlinear effects such as the wavelength-dependent gain fiber amplifiers.
Thermally tunable waveguide devices are attractive for active routing, for compensation of dispersion and for flattening amplifier gain. The physical dimensions and refractive index of light-transmitting materials can be controllably varied by changing the temperature. This permits fine tuning of devices whose operating characteristics depend critically upon index and dimensions. For example, variation of the temperature can be used to tune the wavelength at which Bragg gratings will reflect, the wavelength of peak loss for long period gratings, and even the degree of chirp in a grating.
Thermally tunable Bragg gratings essentially comprise lengths of optical waveguide including periodic perturbations in the refractive index which are spaced apart by a distance which is smaller than the transmitted wavelengths. A current-controlled thin-film resistive heater along the length of the grating can fine-tune the perturbation spacing, and thereby tune the reflected wavelength or the grating chirp. Such devices are described in greater detail in the following four articles which are incorporated herein by reference. (H. G. Limberger, N. H. Ky, D. M. Costantini, R. P. Salathe, C. A. P. Muller and G. R. Fox xe2x80x9cEfficient Miniature Fiber-Optic Tunable Filter Based on Intracore Bragg Grating and Electrically Resistive Coating,xe2x80x9d IEEE Photonics Technology Letters 10(3), March 1998, p.361-363; G. R. Fox, C. A. P. Muller, N. Setter, D. M. Costantini, N. H. Ky and H. G. Limberger, xe2x80x9cWavelength Tunable Fiber Bragg Grating Devices Based on Sputter Deposited Resistive and Piezoelectric Coatings,xe2x80x9d Journal of Vacuum Science and Technology Part B, 15(3), May/June 1997, p. 1791-1795; B. J. Eggleton, J. A. Rogers, T. A. Strasser, xe2x80x9cOptical Grating Devices With Adjustable Chirp,xe2x80x9d U.S. patent application, Ser. No. 09/183,048, is now U.S. Pat. No. 6,275,629; K. R. Amundson, B. J. Eggleton, R. J. Jackman, J. A. Rogers and T. A. Strasser, xe2x80x9cThermally Adjustable Optical Fiber Grating Device With Packaging For Enhanced Performance, U.S. patent application Ser. No. 09/252,708 filed Feb. 2, 1999, is now U.S. Pat. No. 6,370,300).
Thermally tunable long period gratings similar to tunable Bragg gratings except that the perturbations are spaced apart by a distance which is larger than the transmitted wavelengths. They operate by controlling the loss of different modes of transmitted light. Thin film resistive heaters can control the wavelength of greatest loss or the grating chirp. Such devices are described in greater detail in the following two articles which are incorporated herein by reference. (D. M. Costantini, H. G. Limberger, R. P. Salathe, C. A. P. Muller and S. A. Vasiliev, xe2x80x9cTunable Loss Filter Based on Metal Coated Long Period Grating,xe2x80x9d Optical Fiber Conference Proceedings, 1998 and A. A. Abramov, B. J. Eggleton, J. A. Rogers, R. P. Espindola, A. Hale, R. S. Windeler and T. A. Strasser, xe2x80x9cElectrically tunable efficient broadband long-period fiber grating filter,xe2x80x9d IEEE Photonics Technology Letters, 11(4), 445 (1999).
As the desire for increased bandwidth requires additional and more closely spaced signal channels, the demands for stability of thermally tuned components becomes increasingly stringent. Because these components rely on heating, their operation can be affected by changes in ambient conditions such as temperature, humidity and air flow. Their operation can even be affected by the thermal mass of bulky temperature sensors. Even if the components are well isolated from their surroundings, they are sensitive to slight fluctuations in the thermal properties of their surroundings, and the more efficient they are, the greater the sensitivity. This sensitivity can produce undesirable changes in the transmitted signals.
Accordingly there is a need for stabilized thermally tunable optical devices.
In accordance with the invention, a thermally tunable optical waveguide device is stabilized against ambient changes. Specifically, a feedback signal derived from a temperature-dependent resistance is used to stabilize the device with respect to ambient changes that could otherwise alter the temperature. Specific embodiments include resistance-heated tunable gratings.